Well I am gonna skip a few years straight to where the first signs of fish were, this is where my question can be found. Ok well after a while one type of fish acquired the first characteristics of landanimals, it had lungs and a ribcage to help its organs against the force of gravity. But what caused them to change? like what caused the fish to change inside it?? like its not just one day they wake up and all of a sudden they have lungs is it?? so what i am really asking is what caused the fish to produce lungs and a ribcage and kidneys and a backbone?? why did the fish get lungs??, do they get it by telling thier body or something?? idk I hope u understand what I am asking cuz we are learning evolution in my biology class but the movie we are watching failed to say how the fish got their charactersitcs of land animals. I was hopping you could help PLEASE!!

Leah wrote:

Well I am gonna skip a few years straight to where the first signs of fish were, this is where my question can be found. Ok well after a while one type of fish acquired the first characteristics of landanimals, it had lungs and a ribcage to help its organs against the force of gravity. But what caused them to change? like what caused the fish to change inside it?? like its not just one day they wake up and all of a sudden they have lungs is it?? so what i am really asking is what caused the fish to produce lungs and a ribcage and kidneys and a backbone?? why did the fish get lungs??, do they get it by telling thier body or something?? idk I hope u understand what I am asking cuz we are learning evolution in my biology class but the movie we are watching failed to say how the fish got their charactersitcs of land animals. I was hopping you could help PLEASE!!

I think that's a very fair question Leah.  The process of evolution is often quickly glossed over, particualrly in schools.

The fish did not tell their bodies to grow lungs.  As I'm sure you realise, that is a rather odd idea.

Instead, what happens is all by chance, and occurs over millions of years.  Imagine a pond drying up, located near another pond.  In the same way some people are stronger or taller than others, the fish with the strongest fins, and able to last longest out of water will be able to survive and drag themselves to the other pond.  Here they will survive and have offspring.  In the same way tall parents will often have tall children, and parents with dark hair will often have children with dark hair, these stronger fish, able to last longer out of water will have children able to do the same.  When the time comes that the pond dries out again, only the strongest offspring with the ability to last longer out of water will survive.

This is what we call selection:  A character or 'trait' is selected by the conditions of the environment, and it becomes common and exaggerated in the group of animals, because only those with the trait survive long enough to have children, and then the children inherit these traits.

You can possibly imagine something along these lines happening until we have something resembling a modern day lungfish (http://en.wikipedia.org/wiki/Lungfish) which can breath by passing air over it's gills as opposed to water.

From fish, evolution created amphibians.  Yes they were land animals, but they still need water to keep their skin moist and to breed in.  And this is when we find fossils like Acanthostega (http://en.wikipedia.org/wiki/Acanthostega - note that it's body is still similar to a fish, though it's fins have started to separate into fingers and toes).

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A pretty contrived hypothetical example would be if all the people over 5'6" in the world disapeared.  short parents will usually have short children, so the next generation would mostly be short (there would be a few tall people in there as there are always exceptions).  if all the people over 5'5" then disapeared, the next generation would be slightly shorter.

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So to answer the crux of your question, "what causes fish to grow lungs?" the answer is: chance, variation, and selection by environmental conditions over millions of years.

Last edited by Peter Falkingham (18th Mar 2008 09:29:18)

Hi Leah, it is also worth remembering that traits don't appear all at once. The fish in the small pond probably had a difficult time with silt getting in their gills and the water being low in oxygen, so even before they have to make a break for new ponds they may have been adapting to gulp air from the surface, becoming better at it as evolution favoured the best survivors. This ability would become an added bonus later on, when their ancestors have to start moving to new places over land. This is sometimes called a preadaptation - it didn't evolve to allow the "next step" to occur, but because it already existed it became an even greater benefit later on.

When you are thinking about something evolving try not to think about an "end product", always try to think about the advantages each variation offers. A spine is great for offering a strong but flexible support down the middle of an animal. It also turned out to be good for providing a support mechanism on land, but that is not why the spine first evolved.

A rather good point I somewhat obscured by attempting to keep my example small and manageable!  Thanks Paolo.

It is also worth adding the a whole bunch of modern fish can breathe in air either partilly or fully under the right circumstances such as mudskippers, lungfish and siamese fighting fish as they either have a proto-lung (like the lungfish) or a modified swim bladder that can take oxygen from it as they gulp in air (like the fighter) or just by passing air and not water over the gills (like skippers).

Almost all fish will gulp at air when stuck in water that has little oxygen becuase although they are inefficient at extracting the oxygen from the sir directly, it is still better that what is availble in the water (you will often see fish doing this in summer as warmer water holds less dissolved air than cold). So even among modern fish you can see some that are better adapted for certain low oxygen conditions and the evolution of soemthing cloer to a lung with enough selective pressure.

And an analogy I have used before: if it seems strange that evolution always seems to throw sixes, remember that evolution is variation plus selection.  Variation throws dice and gets all kinds of numbers, but selection only keeps the sixes, ruthlessly discarding the rest.